Promethium (chemical symbol Pm, atomic number 61) is a metallic element that is a member of the lanthanide series of chemical elements. All of its isotopes are radioactive. As a source of beta radiation, it is used in thickness gauges and in light signals in which a phosphor is activated by the beta rays. It is also used in a type of nuclear battery. In the future, it may be used as a portable X-ray source, or as a source of auxiliary heat or power for space probes and satellites.

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Occurrence

Promethium can be formed as a product of uranium fission. Only trace amounts can be found in naturally occurring uranium ores. For example, a sample of pitchblende was found to contain promethium at a concentration of four parts per quintillion (1018) by mass.[1]

Promethium has also been identified in the spectrum of the star HR 465 in Andromeda, and possibly HD 101065 (Przybylski's star) and HD 965.[2]

History

The existence of promethium was first predicted by Bohuslav Brauner in 1902; this prediction was supported by Henry Moseley in 1914, who found a gap for a missing element which would have atomic number 61, but was unknown (however, Moseley of course had no sample of the element to verify this). Several groups claimed to have produced the element, but they could not confirm their discoveries because of the difficulty of separating promethium from other elements. Promethium was first produced and proved to exist at Oak Ridge National Laboratory (ORNL) in 1945 by Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell by separation and analysis of the fission products of uranium fuel irradiated in the Graphite Reactor; however, being too busy with defense-related research during World War II, they did not announce their discovery until 1947.[3] The name promethium is derived from Prometheus in Greek mythology, who stole the fire of the sky and gave it to mankind. The name was suggested by Grace Mary Coryell, Charles Coryell's wife, who felt that they were stealing fire from the gods.

In 1963, ion-exchange methods were used at ORNL to prepare about ten grams of promethium from nuclear reactor fuel processing wastes.

Today, promethium is still recovered from the byproducts of uranium fission; it can also be produced by bombarding 146Nd with neutrons, turning it into 147Nd which decays into 147Pm through beta decay with a half-life of 11 days.

Promethium has one semi-stable isotope (145) that is a soft beta emitter. It does not emit gamma rays, but beta particles impinging on elements of high atomic numbers can generate X-rays. Promethium salts luminesce in the dark with a pale blue or greenish glow due to their high radioactivity.

Isotopes

36 radioisotopes of promethium have been characterized, with the most stable being 145Pm with a half-life of 17.7 years, 146Pm with a half-life of 5.53 years, and 147Pm with a half-life of 2.6234 years. All of the remaining radioactive isotopes have half-lives that are less than 364 days, and the majority of these have half lives that are less than 27 seconds. This element also has 11 meta states with the most stable being 148Pmm (T½ 41.29 days), 152Pmm2 (T½ 13.8 minutes) and 152Pmm (T½ 7.52 minutes).

The isotopes of promethium range in atomic weight from 127.9482600 u (128Pm) to 162.9535200 u (163Pm). The primary decay mode before the longest-lived isotope, 145Pm, is electron capture, and the primary mode after is beta minus decay. The primary decay products before 145Pm are neodymium (Nd) isotopes and the primary products after are samarium (Sm) isotopes.

Stability of promethium isotopes

Beside technetium, promethium is one of the two elements with an atom number less than 83 that solely has unstable isotopes, which is a rarely occurring effect of the liquid drop model and stabilities of neighbor element isotopes.

In a nuclear battery in which photocells convert the light into electric current, yielding a useful life of about five years using 147-Pm.

Promethium(III) chloride (PmCl3) mixed with zinc sulfide (ZnS) was used for a while as luminous paint for watches after radium was discontinued. Still used for some luminous paint applications.

Possibly in the future as a portable X-ray source, as an auxiliary heat or power source for space probes and satellites.

Precautions

Promethium must be handled with great care because of its high radioactivity. In particular, promethium can emit X-rays during its beta decay. Note that its half-life is less than that of plutonium-239 by a factor of multiple thousands to tens of thousands. Promethium has no biological role.

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